The efficient fragmentation and breaking of rock by means of explosive charges demands considerable skill and expertise. In most mining operations explosive charges, including boosters, are placed at predetermined positions near or within the rock, for example within boreholes drilled into the rock. The explosive charges are then actuated via detonators having predetermined time delays, thereby providing a desired pattern of blasting and rock fragmentation. Traditionally, signals are transmitted to the detonators from an associated blasting machine via non-electric systems employing low energy detonating cord (LEDC) or shock tube. Alternatively, electrical wires may be used to transmit more sophisticated signals to and optionally from electronic detonators. For example, such signaling may include ARM, DISARM, and delay time instructions for remote programming of the detonator firing sequence. Moreover, as a security feature, detonators may store firing codes and respond to ARM and FIRE signals only upon receipt of matching firing codes from the blasting machine. Electronic detonators are often programmed with time delays with an accuracy of the order of about 1 ms.
The blasting systems discussed above employ physical connections between the detonators to be fired and a control unit such as a blasting machine. Typically, detonators are placed at the blast site in association with explosive charges, and connected to surface harness wires (e.g. wires, shock tubes, detonating cords or the like). Detonators present at the blast site may be selectively actuated in groups. In this way, a blast may be conducted in two or more stages. Care must be taken to ensure that later-stage detonators, their associated charges, and their connections to harness wires are not disrupted or suffer damage from explosive forces derived from earlier-stage firing. Nonetheless, it is possible to selectively actuate one group of detonators before other groups of detonators are actuated at a blast site. In such blasting systems selective, staged actuation of groups of detonators may be achieved via fairly simple means. For example, those detonators that are required to actuate for a particular stage of a blast may be connected to the harness wire(s) whereas those not required may remain unconnected or be disconnected from the harness wires. Alternatively, where multiple harness wires are present, each group of detonators may be connected to a different harness wire, with a command signal to FIRE each group transmitted via a different harness wire as desired.
Recent years have seen the development of wireless blasting systems for use in blasting rock. Such systems present significant advantages over more traditional wired blasting systems. By avoiding the use of physical connections between detonators, and other components at the blast site (e.g. blasting machines) the possibility of improper set-up of the blasting arrangement, such as improper ‘tieing-in’ of detonators, is reduced. Wireless blasting systems offer excellent potential for automated establishment at the blast site. For example, robotic systems may be more readily deployed for placement of wireless detonator assemblies and associated explosive charges at a blast site, since the complications of trailing wires (and the need to connect explosive devices to such wires at the blast site) are completely avoided. Wireless blasting systems, and corresponding methods employing such systems, are disclosed for example in international patent publication WO06/047823 published May 11, 2006, WO06/076777 published Jul. 27, 2006, WO06/096920 published Sep. 21, 2006, and WO07/124,539 published Nov. 8, 2007, all of which are incorporated herein by reference.
Nonetheless, the development of wireless blasting systems, and components thereof, presents a formidable technological challenge. In just one example, selective control and firing of wireless detonators in pre-determined groups (as discussed above in the context of wired blasting systems) is not simple to achieve since there are no harness wires present for selective connection of the detonators. Hence there is a need in the art for methods of blasting that permit selective control of detonators and their corresponding wireless detonator assemblies, in the context of wireless blasting systems for mining.